Like Mileybun mentioned, it's important to address why the poopy butt is occurring as well, if you aren't already working on that. Usually it's mushy cecotropes caused by too many carbs and insufficient fiber/hay in the diet, but occasionally there can be other causes like other dietary sensitivities, dental issues, obesity, arthritis, liver problems, etc. If all poop is runny or mushy, including fecal balls, that's true diarrhea and considered an emergency.
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If in doubt about potential medical issues, it's best to consult with a knowledgeable rabbit vet. Near your location, Aidan Raftery at Ashleigh vets is supposed to be a very good rabbit vet. You can also check the list for other recommended rabbit vets.
Creating better tomorrows for all pet rabbits Recommended Rabbit-Friendly Vets A vet with specific knowledge of rabbits is exactly what you need when choosing the right medical care. With such different physiology to cats and dogs, a real enthusiast is a good call for your rabbits’ health. So...
rabbitwelfare.co.uk